I am slightly confused about how size categories for creatures in Pathfinder are determined.

The Rat is considered size Tiny.
The Squirrel is considered size Diminutive.

The squirrel is stated as being about a pound.
The largest species of rat (the black rat) is 3.9-12 ounces.

A fox is in the Tiny size category. The average size of a fox is:
Height: 14 – 20 in. (Adult, At Shoulder)
Length: 18 – 35 in. (Adult, Without Tail)
The average adult is about 20lbs. They can be as large as 35 pounds.

The average cat runs about even with the smaller end of the fox spectrum- and is described in the Bestiary as 5-15 pounds.

The Dire Rat, according to the Bestiary, grows up to 2 feet long and weigh up to 25 pounds.

Dire Rats are size Small.

I feel like there is a discrepancy and I am confused about how these categories are determined when writing Bestiary entries.

Honestly, I think this has more to do with Fantasy Tropes and the history of RPGs using Rats in general in the place of monsters in many situations.

Leaving them more realistically as Diminutive like the Squirrel probably makes more logical sense if they were trying to "import" Earth rats, but I think that what we are dealing with here is simply that Golarion rats are just bigger than our native version.

I've owned several pet rats over the years and while I must say they can get big, and there are even some wild rats that get even larger, they in no way compare to how CHONK the Squirrels who get into my garbage are.

Seems to me like a narritive choice to make them more dangerous and lessen the drastic jump of Dim>Small for a normal to a Dire rat.

Source? I'm no mammologist, but a quick glance at Ye Olde Wiki purports that Norwegian brown rats are larger. Regardless, real world size, especially present day real world size, often has little bearing on the size of a creature in Pathfinder. (For fun, compare the Pathfinder giant gecko with the real world species.)

The good news is that it usually doesn't matter; for most purposes, size category is all that is needed to decide how things work.

Edit: Ha, never noticed that Pathfinder cats are larger than foxes . . . somehow. *shrug* Maybe they're more like ocelots or lynxes?

Be careful around those foxes. With 9 strength, they can drag off people who weigh up to 225 lbs.

You forgot that they are quadrupeds. Make that 337 lbs.

Foxes are tiny, it's actually 169 lbs.

Nope. Maximum weight that a medium size character with strength 9 can carry is 90 pounds. The maximum weight that such a character can drag is five times that, or 450 lbs. The multiplier for a tiny quadruped is x3/4, so the maximum weight that a fox can drag is 3/4 x 450 lbs., or 337 lbs.

Nope. Maximum weight that a medium size character with strength 9 can carry is 90 pounds. The maximum weight that such a character can drag is five times that, or 450 lbs. The multiplier for a tiny quadruped is x3/4, so the maximum weight that a fox can drag is 3/4 x 450 lbs., or 337 lbs.

TIL I've been doing it wrong, I've been multiplying it by the Bipedal sections first (I thought that was an initial size multiplier), and then the quadrupedal, never realizing I just do the quadrupedal. Thanks.